The Labour party is claiming a record-breaking surge in its membership since the election amid signs that disgruntled Liberal Democrats are flocking to the party.

Almost 10,000 people have joined the party since the close of the polls. The post-election boost represents a 6% rise in its overall membership, after years of dwindling numbers willing to commit to the party.

Some of the new members claim they have signed up as protest at the decision of the Liberal Democrats to join the Conservatives in a coalition. But despite the unease of its own activists the Liberal Democrats are also boasting hundreds of new members in the last week.

The sharpest rise in new Labour members came after details emerged of the coalition agreement. During the course of yesterday alone, as Nick Clegg and David Cameron held their joint press conference, 4,211 joined Labour.

"Unprecedented numbers of people joining the Labour Party. 1 every 15 seconds at the moment," the party's Twitter feed said.

One of the new members is Layla English. "I joined because I was so angry about what's been going on for the last couple of days, particularly about the way the Liberals have conducted themselves," she said.

"Some of their polices were more leftwing than the Labour party, and they've just abandoned all that for a place at the top table with the Conservatives."

She added: "I've always voted Labour, but they now need more support more than ever because of they are in financial difficulties. It made me feel better that I was doing something about it – I can't go and vote again."

Newly signed-up members are asked to state their reasons for joining. A selection of these comments, published on the Labour party website suggest many have joined as reaction to Liberal Democrats' role in the new coalition. Some said they felt betrayed by the Lib Dems.

"I was appalled that Nick Clegg has backed the Tories," Amy from Chesterfield said.

"I made the mistake of voting for the Liberal Democrats, first, and last time," Scott from Manchester is quoted as saying. Similarly Mark from Newport told the party: "Frustrated at tactically voting Lib Dems who then form an alliance with the Tory party. Will never vote Lib Dem again."

But the Liberal Democrats have also recorded an increase in party members since the election.

A spokesman said 1,500 had joined since polling day, including 400 since the formation of the coalition. The party currently has just over 60,000 members. Both Labour and Liberal Democrats spokesmen said they did not have figures on how many members of their respective parties had left since the election.

The Labour spokesman claimed that five people per minute are currently signing up to Labour.

Before the election on a typical day the party only attracted around 20 new members a day. This rate fell well short of the numbers allowing their membership to lapse.

The party struggled to attract and retain new members while in government. Membership peaked at 405,000 in 1997 in the aftermath of Tony Blair election. But 10 years later it had fallen back to 176,891, thought to be its lowest level since it was founded.

A spokeswoman for the Conservatives said it does not publish membership figures, and would not release details of how the figures had changed since the election.